This invention relates to current supply circuits and, more particularly, to integrated circuits (IC) capable of producing currents having regulated magnitudes and predetermined temperature characteristics.
There are many circuit and system applications that require current supplies or sources for providing currents having predetermined temperature coefficients (TC) and regulated magnitudes which are independent of supply voltage. More particularly, it is sometimes desirable to utilize a current supply circuit providing a current with a magnitude that has a positive TC that varies directly with absolute temperature. The current can be exploited to cancel the negative TC inherent in the PN junctions of a differential pair of transistors, for instance, so as to enable the gain of a differential amplifier comprising the differential pair of transistors to remain substantially constant with temperature changes. Since IC's generally can include many such differential amplifiers, it may require a current supply circuit of the above described type that can provide a plurality of such currents each having a predetermined magnitude and temperature coefficient associated therewith. Another use for such thermal current sources is in conjunction with other circuitry to provide a regulated output voltage having a known TC. For example, the thermal current can be utilized to produce a voltage across a resistor having a positive TC which is then placed in series with the negative TC base-to-emitter voltage of a NPN transistor to provide a zero TC output voltage. These types of voltage regulators are sometimes referred to by those skilled in the art as bandgap voltage regulators.
In general these prior art thermal current sources utilize a pair of interconnected NPN transistors which are operated at different current densities to produce a base-to-emitter voltage difference therebetween which has a positive TC. The voltage difference is used to set the current in the emitter of one of the transistors which varies with temperature in the same manner as the difference voltage. This thermal current then establishes a thermal collector current through the transistor that can be utilized as mention above.
However, a problem exist in most, if not all, of these types of thermal current sources which relates to setting the current in the emitter of the transistor. Since the emitter current also includes the base current of the transistor whereas the collector current does not, there exist an error therebetween known as beta or alpha errors, as understood. Another source of error in prior art circuits arises if the collector-emitter voltages of the two transistors are not well matched. In this case, variations in the power supply voltage can produce errors in the output current. Moreover, since an output current is typically taken at the collector of one of the two transistors, the collector currents become unequal. Consequently the prior art reference circuits tend to function improperly when required to drive multiple output loads.
Hence, a need exists for an improved integrated thermal current source circuit that overcomes the problems of prior art thermal current source circuits of the type described above.